Fix GFF3 parser to handle trailing FASTA

[benwbooth]

The GFF3 spec allows trailing FASTA sequences to be put into GFF3 files with a ##FASTA line preceding them. ADAM's GFF3 parser should either stop parsing the GFF3 file if the ##FASTA line is encountered, or possibly also return a NucleotideFragmentRDD with the parsed FASTA included somehow.

[heuermh]

Hello @benwbooth!

Yes, loadFeatures should not choke on a GFF3 file if it contains FASTA sequences. We don't really have any methods in our API that return more than one GenomicDataset types from the same file, so we'd probably want to add GFF3 support to loadSequences. Let me know what you think.

[benwbooth]

That seems sensible. Thanks!